Remember that the freezing point of a solution is decreased by having anything dissolved in it. When a soft drink is opened and CO₂ rushes out, less CO₂ is present in the solution, causing the freezing point of the solution to increase. So at a constant temperature, the increase in freezing point causes the soda to freeze. If you haven’t seen this happen, you can try to make it happen by placing a soda in the freezer for a while, but be careful not to leave it in too long or it might explode!